An active-matrix substrate for use in a liquid crystal display device and other devices includes switching elements such as thin-film transistors (which will be simply referred to herein as “TFTs”), each of which is provided for an associated one of pixels. As such switching elements, a TFT that uses an amorphous silicon film as its active layer (and will be referred to herein as an “amorphous silicon TFT”) and a TFT that uses a polysilicon film as its active layer (and will be referred to herein as a “polysilicon TFT”) have been used extensively.
Recently, people have proposed that an oxide semiconductor be used as a material for the active layer of a TFT instead of amorphous silicon or polysilicon. Such a TFT will be referred to herein as an “oxide semiconductor TFT”. Since an oxide semiconductor has higher mobility than amorphous silicon, the oxide semiconductor TFT can operate at higher speeds than an amorphous silicon TFT. Also, such an oxide semiconductor film can be formed by a simpler process than a polysilicon film, and therefore, is applicable to even a device that needs to cover a large area.
Patent Documents Nos. 1 and 2 disclose TFTs with a bottom gate structure that uses an oxide semiconductor. Specifically, in the TFT disclosed in Patent Document No. 1, a metal oxide layer is arranged between an oxide semiconductor layer and source/drain electrodes in order to make them contact with each other more closely. Meanwhile, Patent Document No. 2 proposes that an opaque layer be provided closer to the viewer than an oxide semiconductor TFT is in a display device including an oxide semiconductor TFT with a bottom gate structure as its switching element (FIG. 6 of Patent Document No. 2).